This invention relates generally to turning machines and specifically to a new and unique CNC turning machine which is adapted for complex turning of parts such as pistons.
An example of a known turning machine is illustrated in U.S. Pat. No. 3,869,947, commonly assigned. In the turning machine disclosed in thar patent, a part to be turned, for example a piston, is suitably chucked and rotated about its axis at an appropriate speed. A cutting tool is arranged to make a pass along the rotating part and machine the outer part surface, i.e. the piston skirt. As the cutting tool is making its axial pass along the part, the cutting tool's radial position is continuously correlated with the rotation of the part to produce a desired shape. This correlation is achieved by a cam and follower system. Such a system is capable of imparting both eccentricity and taper to the part, i.e. complex turning. In other words in the case of a piston, the system is capable of producing either straight or tapered skirts of either circular or elliptical cross section, depending upon the desired shape.
Such a turning machine is well suited for making large numbers of identical parts. However if different shaped parts are to be turned, the machine must be shut down for change-over. When such shut-down occurs, the machine is removed from productive use, and where precision parts are involved, as is usually the case, care must be taken to ensure that the new cam and the follower produce the desired precision. The amount of set-up time for the new cam also adds to the total machine downtime.
Turning operations of the type conducted on piston skirts typically involve relatively high rotational speeds for the pistons. In the cam and follower type system described above, the dynamics of the machine and mechanism can limit the ability of the follower to track the cam. Hence minor changes in the cam surface shape may be difficult to follow, and there is necessarily a limit to the ultimate precision with which parts can be machined by such an apparatus for a given production rate.
Another prior art approach to controlling the radial position of the tool head in a piston turning operation is disclosed in U.S. Pat. No. 4,203,062, issued May 13, 1980 to Bathen for "Machine Tool Control Systems." The Bathen system employs computer numerical control with a feedback loop which compares a position signal representing the present position of the tool to a programmed position signal to produce an error signal which controls energization of a linear motor driving the tool.
The present invention is directed to a new and improved turning machine which possesses a number of important advantages over prior machines.
One important advantage is that the present invention eliminates the mechanical cam and follower type system by using numerical input data to define the part shape. This data is acted upon by a CNC system which generates appropriate commands to control the cutting tool position at all times during turning. Hence a turning machine embodying principles of the invention is not limited by the mechanical dynamics of the prior cam and follower systems which established an ultimate limit to the machine's capabilities.
Because the control data is embodied in electronic form in the practice of the present invention rather than as a mechanical model like a cam, there is no elaborate mechanical change-over required when part shape is to be changed. Rather the CNC is provided with a new part program for the new part, and it automatically acts upon the new part program data to issue appropriate commands for control of the cutting tool.
Moreover, with the elimination of the mechanical cam and follower, the present invention affords the opportunity for attaining even higher degrees of precision in the high speed turning of parts.
Not only is the versatility of a turning machine significantly enhanced since it can handle many different part sizes, but with improved efficiency and precision potentials, the opportunity for significant productivity gains is also presented by the present invention.
The general idea of applying a CNC system to a cutting tool is of course known. For example CNC lathes are representative commercial products. However in the context of a high speed turning apparatus such as a piston turning machine, the application of CNC technology has heretofore been deemed impractical because of inherent mechanical limitations in mechanism for positioning the cutting tool.
Consider a situation where a part is to be turned at say several thousand RPM and is to be provided with an elliptical cross sectional shape. The cutting tool must make two reciprocations radially of the part for each complete revolution of the part. In the case of a piston rotating at 2400 RPM, this means that the cutting tool is required to execute precisely controlled linear oscillations at a frequency of 80 hertz. For example if it is assumed that the acceleration of the tool is required to follow a 0.007 inch radial displacement at this speed, the acceleration amounts to 37 feet per second per second. In order to achieve this magnitude of response, the mass associated with the oscillating cutting tool must be small. Yet at the same time that the mass, including the cutting tool, is executing this oscillatory motion, they are being subjected to a load imposed by the interaction of the cutting tool with the rotating part. The requirements of minimizing the mass associated with the cutting tool in order to attain a satisfactory response at the expected oscillatory frequencies and of accurately linearly guiding same with minimum static and dynamic friction, are seemingly inconsistent with requirements that the cutting tool and its associated mass be sturdily constructed and supported to react the loads imposed on them without undesired effects such as tool chatter and/or deflection so that the desired contour of the part can be achieved. Moreover, since many parts are of complex contour including an axial taper, such taper usually has to be taken into account as well.
Accordingly, another aspect of the present invention involves a new and unique construction for the mechanical mechanism which oscillates the cutting tool. Among the new and unique features are the prime mover which is utilized to impart oscillatory motion to the cutting tool, the construction of the cutting tool carriage, and the arrangement for guiding the carriage on a head.
In the preferred embodiment of the invention the prime mover comprises a linear motor, sometimes referred to as a voice coil motor. This prime mover has a low inertia armature for fast response, yet it is capable of precise movement while exerting ample force to counter cutting loads imposed when the cutting tool interacts with a part being turned. The cutting tool carriage is operated by the motor armature. A sturdy, yet low friction, mounting of the carriage on the head also assists in reacting the cutting loads while enabling the desired oscillatory action to be obtained so that accurate parts are consistently produced.
Another aspect of the invention involves the cooperative relationship between the CNC system and certain mechanical mechanisms of the machine. A portion of the CNC operation is devoted to a closed loop control with the part being turned whereby the relative axial position of the part to the cutting tool and the rotational position of the part about the axis of its rotation are precisely controlled and known at all times. The CNC acts upon the part program in conjunction with the aforementioned closed loop control to issue correlated commands for use in controlling the voice coil motor, and hence the radial oscillation of the cutting tool. These commands are transmitted by via a high speed data link to a position profile computer which translates the commands into an appropriate form for causing the voice coil motor to produce the double oscillation of the cutting tool per each revolution of the part when an elliptical contour is being machined.
The position profile computer is a system dedicated to the radial positioning of the cutting tool, and it forms a portion of a closed loop control for the cutting tool position. Associated with the voice coil motor and carriage, are various sensors which provide feedback signals to this latter closed loop control. These are all operatively related such that digital data from the CNC and feedback signals from the various sensors are appropriately processed to produce a control current for the voice coil motor which produces the desired oscillation of the cutting tool.
Accordingly the dedicated system comprises digital circuit components performing digital calculations. It also has digital-to-analog devices organized and arranged to act upon certain digital commands to produce an appropriate analog control current for the voice coil motor. As will be seen leter, there are particular relationships involved in this closed loop control which are advantageous in causing the tool to faithfully follow the CNC digital commands.
The foregoing features, advantages, and benefits of the invention, along with additional ones, will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings. The drawings disclose an exemplary, presently preferred embodiment of the invention according to the best mode contemplated at the present time in carrying out the invention.